Presently existing sensors comprise an elongate housing which is fixed substantially vertically in the casing containing the liquid, said housing having top and bottom openings and havaing a heat sensitive resistor wire disposed longitudinally therein with the ends of the wire being electrically connected to a feeder and processor circuit enabling the level of the liquid to be monitored at any moment.
An example of a feeder and processor circuit which is well adapted to monitoring an oil level is illustrated in French Pat. No. 2,367,276 (corresponding to U.S. Pat. No. 4,163,391). In general, a current pulse is applied during a predetermined duration (for example, for about two seconds), and the voltage across the ends of the resistive wire is measured at two instants, e.g. at the beginning and at the end of the period during which the current pulse is applied. The difference .DELTA.U between the measured voltages is compared with a predetermined threshold differnece .DELTA.U.sub.O, and an ON/OFF alarm is triggered when said threshold is past, or an analog output is provided as a function of .DELTA.U.
Such sensors make use of a heat sensitive resistive wire which is V- or W-shaped, thereby providing a considerable length of wire inside the housing even though the volume of the housing remains small. However, there is a lower limit on miniaturization of the sensor due to capillarity phenomena which leave a certain quantity of liquid around the wire, particularly around the middle point of a V-shaped wire or around the bottom points of a W-shaped wire, thereby spoiling the measurement.
In the preferred application of the present invention, i.e. when monitoring the level of oil in the engine or the gearbox of a motor vehicle, there is another factor which interferes with measurement, and that is the unavoidable presence of vibrations in the liquid-containing casing. Because of this, oscillation in the arms of the heat sensitive resistive wire reduce measurement accuracy by accentuating the effects of capillarity at the points in the V- or W-shaped wires, and additionally such oscillations fatigue the wire and may cause it to break.
Attempts have been made to mitigate this drawback by using longitudinal springs to increase the tension in the wire, however, this has the effect of increasing the size of the sensor. One such sensor is described, for example, in published European patent application No. 0,043,312.
Preferred embodiments of the present invention provide a liquid level sensor which is robust, reliable, and less sensitive to disturbances than present sensors, and in particular which has better vibration performance.
Preferred embodiments of the present invention also make it possible to provide a sensor which is particularly compact without disturbing the accuracy of its measurements, and this may lead to other functioal members being included in the sensor housing, e.g. a thermistor.